The present invention relates to a wireless cellular communications and more particularly to a frequency reuse scheme and a corresponding frequency hopping scheme in an hexagonal wireless cellular communication network.
In a wireless cellular communication network, a limited number of frequencies are available for communicating. Each frequency must be reused in different cells of the network causing interference between different users using the same frequency. An appropriate frequency reuse scheme should be applied in the network to reduce the interference and optimize the network capacity.
For example, in a network having hexagonal cells, a simple frequency reuse scheme using seven different frequencies consists in repeating the pattern as shown on FIG. 1 in the whole network. A first base station at the center of an hexagonal cell C1 supplies the whole cell with signals at the frequency f1. This is ensured by a omni-directional antenna at the center of the cell. Six further hexagonal cells C2, . . . , C7 are surrounding the cell C1. Signals at the frequencies f2, . . . , f7 are respectively supplied in the cells C2, . . . , C7. This pattern P1 using seven frequencies is repeated to pave the whole network.
In order to increase the capacity of the network, in particular in urban areas, it is appropriate to subdivide each hexagonal cell in a plurality of sectors, each sector being allocated one of the available frequencies. For example, an hexagonal cell may be subdivided in six 60° angular sectors each comprising a directional antenna. If six frequencies are available in the network, each frequency will be used once in one sector of each cell. Known patterns are disclosed for example in U.S. Pat. No. 5,459,759.
Moreover, frequency hopping can be implemented to further optimize the network. Frequency hopping consists in changing the frequency used in the different sectors of the cell at regular time intervals, called the hop duration. Of course, the frequency hopping scheme must be controlled in the cells and coordinated so that
no sector in the some cell use the same frequency at the same time
the distance between two sectors of different cells using the same frequency at the same time must be kept high enough to minimize interference.
FIG. 2 is a representation of a known frequency hopping scheme used in an hexagonal cell subdivided in six 60° angular sectors. For this purpose six different frequencies are used. In this representation, the frequencies indicated in each concentric circle CC0, . . . , CC5 correspond to the frequency used in the corresponding sector during one time slot. This concentric circle CC0 corresponds to time slot T mod(6), CC1 to time slot T+1 mod(6), CC2 to time slot T+2 mod(6), CC3 to time slot T+3 mod(6), CC4 to time slot T+4 mod(6) and CC5 to time slot T+5 mod(6). Indeed, the indicated frequency hopping sequences are repetitively used for transmitting signals in the different sectors.
On FIG. 2, the sequences used in the different sectors are the following:    for sector S1: frequency hopping sequence f1; f3; f5; f2; f6; f4;    for sector S2: frequency hopping sequence f5; f1; f3; f6; f4; f2;    for sector S3: frequency hopping sequence f3; f5; f1; f4; f2; f6;    for sector S4: frequency hopping sequence f6; f2; f4; f1; f5; f3;    for sector S5: frequency hopping sequence f2; f4; f6; f3; f1; f5;    for sector S6: frequency hopping sequence f4; f6; f2; f5; f3; f1.
The frequency hopping scheme presented in this document is however presenting interference higher than a predefined level in 13% of the whole cell area.
A particular object of the present invention is to provide a frequency reuse scheme and a corresponding frequency hopping scheme adopted to minimize the amount of interference experienced in a wireless cellular communication network having cells subdivided in N identical sectors.
Another object of the invention is to provide a transmitter able to generate signal in accordance with a frequency reuse scheme and a corresponding frequency hopping scheme.